This question was put to me one afternoon by a lady at pony club when Iarrived to pick up my children. The story was a familiar one of a young pony that had beenpurchased in the winter months then suddenly developed the characteristic rubbing andweeping sores of sweet itch the following summer.

Of course the vendor was certain that the pony had shown no signs of the disease theprevious year, which may well be true as the peak age of onset for sweet itch is about fouryears and it can first appear in much older horses.

Initially I was rather taken aback by this abrupt demand to “do something”, yet onreflection I could only agree that it was time something was done about this distressingcondition which affects about 3% of horses and ponies in Britain.

Furthermore, as a vet with an interest in immunology I was in an undeniably good position totake on this task and any chance of backing out was completely lost when my children joinedin with a chorus of “Yeah dad you can fix it”. Fine words; but before one can fix anythingit is essential to understand the exact nature of the problem and this is also largely trueof treating diseases.

Midges are to blame.
Sweet itch is most often an allergic reaction of horses to the bites of midges, which occurin almost every country in the world where horses are kept. Midges are small, only a coupleof millimetres in length and like many insects that feed on blood, only the females do so asthey need the high protein meal to enable them to lay their eggs.

Although the disease has been known for centuries, the first scientific description of thelink with midge bites was made in Australia in the 1950s by a vet called Riek. Riek firstnoticed that the disease only occurred in areas where midges were abundant, then showed thatwhen an extract of midges was injected into the skin of the affected horses, a smallswelling appeared within a few minutes. This type of “acute” reaction is typically seen incertain types of immune “hypersensitivity” or allergy and so Reik concluded that sweet itchwas an allergy to midge bites.

Why do midges spit?
A few days after the pony club incident, I was out in the field checking our ponies, it wasa calm sunny evening and as I watched I could see large numbers of midges, which are mostactive around dawn and dusk or on overcast days when there is less risk of them drying outin the hot sun.

After alighting they crawl down the hair shafts to the skin surface, their mouthparts aretoo short to probe for a blood vessel like their larger cousins the mosquitoes, so they haveto chew their way through the tough outer layers of skin. To assist their efforts theysecrete saliva containing a mixture of enzymes that digest and soften the skin tissue aswell as “vasodilators” to encourage extra blood to flow to the site of the bite and severalfactors that will prevent the blood clotting. A small pool of blood forms just under theskin surface and is then sucked up by the midges.

Once full they make their way back to the ends of the hairs from where, laden with blood andweighing twice as much as when they arrived, they launch themselves into the air. The wholeprocess takes about 15-20 minutes and over the course of an evening a horse may be bitten byhundreds or even thousands of midges each one injecting a small amount of saliva containingforeign proteins into the horse.

The response of the immune system to midge saliva.
When an animal is injected with something foreign like midge saliva, it’s immune systemresponds by making antibodies that can bind on to the foreign proteins. The action ofantibodies is often compared to that of a key for a lock, in that one end is like the keyshandle, while the other end has a unique shape that fits the lock or in the case ofantibodies allows them to bind their target.

Antibodies are made by specialised cells called B-cells. Each individual B-cell isprogrammed to make a unique antibody, an animal like a horse (or human) has millions of B-cells which between them can make millions of different antibodies. But one B-cell on itsown will not make very much antibody, so when a B-cell encounters a foreign substance thatbinds it’s own unique antibody it is stimulated to grow and divide so that in a few daysthere are thousands of them and lots of antibody can be made.

This is how a flu vaccine works, your horse is injected with a small amount of the influenzavirus and those B-cells which make antibodies that can bind to the virus increase in number.Then when a real flu virus turns up there are lots of B-cells primed and ready toimmediately make antibodies that bind onto the virus and “neutralise it” preventing theinfection spreading, without your horse having to go to the trouble of getting ill andwaiting several days for its immune system to catch up.

So, if the immune system of horses with sweet itch is reacting to the saliva of midges, thehorses should have antibodies that will bind specifically to the proteins in midge saliva.The first stage in our research was to identify which midge proteins are important.

There was no problem getting some midges - all that was needed was to catch them in asuction trap as they landed on the horse to feed. The first approach we used to look for theantibodies that bind to midge saliva proteins, was a technique called immuno-histochemistry.First the midges are sliced into very thin sections then placed on a glass slide. The slidesare exposed to serum from horses so that any antibodies in the serum will bind to theirtarget. We then detect the bound horse antibody using a label that produces a red colour.

However, we found that all British horses had antibodies that bound midge saliva; perhapsthis is not surprising as all horses living in Britain will be bitten by midges. Fortunatelywe were able to get some serum from horses living in Iceland, one of the few places in theworld where there are no midges. The serum from Icelandic horses that had never encountereda midge did not stain the midge saliva glands, confirming that their serum contained noantibodies to midge saliva.

A special kind of antibody
But if all British horses have antibodies to midges, why do only some develop sweet itch?The answer lies in understanding more about the immune system and about antibodies. Thinkagain of the antibody being like a key, with one end specially shaped to bind a foreignsubstance and a handle at the other end.

The B-cell can attach a different handle to its antibody for different purposes, forexample, one type of antibody handle is best for binding to an influenza virus andpreventing it from infecting its host. A different type of antibody handle is needed to dealwith a bacteria like the one that causes strangles, in this case the antibodies bind to thebacteria’s surface and their handle enables white blood cells to catch hold of the bacteriathen ingest and destroy them.

The kind of antibody that is important in allergies like sweet itch is called ImmunoglobulinE or IgE for short, its role is in immune responses to parasites like worms. Adult worms ofcourse are very common in horse’s digestive systems but many of the larval stages of wormsinvade the tissues of the horse and some types of worm actually live just under the skin.

To help protect against worms the immune system has to use one of its most powerful weapons,known as the “mast cell”. These mast cells coat themselves in IgE antibodies and lie in waitjust under the skin or in the lining of the intestine. When the IgE binds its target, themast cell releases a cocktail of chemicals that cause a severe inflammatory reaction andattract other immune cells which can injure or kill the parasite.

Unfortunately, mast cells like other weapons of mass destruction can cause a lot ofcollateral damage. Allergies occur when the immune system makes a mistake and mounts an antiparasite response to the wrong thing. For example, in people this could be a food likepeanuts, or in hay fever it is often pollen, and in some cases people even develop andallergy to horse hair. But in horses themselves the commonest allergy is to midge saliva.

When we looked for IgE antibodies that bound to midge saliva glands we only found them inthe serum taken from horses with sweet itch, confirming that this disease is an allergicresponse to midge bites.

What is it in midge saliva that the horse’s immune system reacts to?
The next task in our research, which was funded by The Horse Trust, was to identify all thedifferent proteins in midge saliva. One way of doing this is to isolate the relevant genesthat contain the instructions for making the proteins in midge saliva. In animals, everycell contains DNA that carries the code for making an entire animal, but only those genesthat are needed by a particular cell are switched on.

From midge saliva glands, we isolated the switched on genes which code for the salivaproteins and put them into a special type of bacteria in the laboratory. When grownovernight on a dish of agar gel each individual bacteria forms a colony that will containonly one extra gene from midge saliva. We can then pick each colony of bacteria, isolate themidge gene and read its coded message. By reading lots of coded messages we can work outwhich ones are most common in midge saliva and are most likely to be the genes of theproteins that cause sweet itch.

The second approach is to look at the proteins themselves. Using a combination of methods,proteins can be separated in a polyacrilamide gel according to their size and acidity. Theindividual proteins form spots and each spot is then punched out of the gel and digestedinto fragments. The sizes of the fragments are then measured by a mass spectrometer. Toidentify the proteins a computer program is used that compares the pattern of the fragmentsizes with those that we would expect to find based on the genetic codes. After putting allthis information together we can work out what the commonest proteins in midge salivaare.

We can also investigate which of the spots bind IgE antibodies in the serum of allergichorses. All horses with sweet itch have IgE antibodies that bind midge saliva proteins butindividual horses will recognise a different pattern of spots.

Once the genes or the relevant proteins have been isolated we can put them into cultures ofinsect cells which will make the protein in an identical way to a midge saliva gland. Eachculture can be over a litre in size and only makes one midge protein so we can produce apure protein equivalent to the contents of several million midges’ saliva glands.

What are we planning to do with all that protein from midge saliva?
We can in theory use it to re-programme the immune system of an allergic horse to act likethat of healthy horses. First we need to better understand why only some horses immunesystem reacts with an allergic response. Although Icelandic horses do not get sweet itch inIceland where there are no midges, when brought to mainland Europe, over one in fourIcelandic horses may eventually develop the condition.

So are Icelandic horses as a breed genetically more likely to get sweet itch? Scientists andvets from Iceland and European countries have looked at this in detail and the answer is no.Although there are genes in some horses that make them more likely to get sweet itch, theseare not more common in Icelandic horses compared to other breeds and Icelandic horses bornon mainland Europe do not get sweet itch any more often than other horse breeds.

Other studies on the development of the foal’s immune system have shown that they do notmake IgE antibodies until they are about 6 months old, we think that when foals are exposedto midges before this age their immune system usually becomes programmed not to make IgEantibodies to midge bites. But Icelandic horses first exposed to midge bites as adults arevery susceptible to developing sweet itch because as foals their immune system was notprogrammed to ignore midge bites.

A case of mistaken identity.
Remember how the B-cell attaches a different handle to its antibodies, but how does it knowwhat kind of handle it needs to use? Well the B-cell is told what kind of handle to use bychemical messages sent by another group of immune cells called T-cells. So how do the T-cells know what chemical messages to send?

The T-cells are told what to do by yet another type of cell called a dendritic cell whichdetects foreign substances and can tell if they are from bacteria, from viruses or are ofparasite origin. This seems to be the root of the problem, the dendritic cells recognisebacteria because they are made of different materials, and when cells are infected byviruses they send out distress calls that alert the dendritic cells to the virus’s presence,but what about parasites?

A migrating parasite has to break down the tissue, to do this it secretes enzymes similar tothose the midge uses to break down the skin and like midges the migrating parasites alsoreleases factors that can interfere with blood clotting. The horse’s immune system “detects”these effects and interprets them as an invading parasite when in fact it is only a midgebite. So it looks like sweet itch is a case of mistaken identity leading to the wrongmessages being sent down the chain of command to the B-cell which responds by making IgEantibodies which trigger an allergic reaction.

Scientists at the Veterinary School in Berne analysed the chemical signals made by theimmune system of horses with sweet itch, and showed a clear difference in the messages madeby the T-cells from healthy horses responding to midge saliva and the T-cells from horseswith sweet itch.

Can we re-program the immune system of horses with sweet itch to be like that of healthyhorses?
The term immunotherapy is used to describe treatments that can be used to re-program theimmune system of people (or animals) with allergies; usually this involves repeatedlyexposing the immune system to small amounts of the allergen. Originally this was done bydaily injections but it is unlikely that this method would be suitable for use in horses.There are several newer ways that re-programming could be attempted but it will take sometime to carefully work out how this can be safely done.

For example, would mixing the midge proteins with some parts of bacteria fool the immunesystems chain of command into giving orders that divert the B-cells from making IgE. Orcould feeding midge protein convince the immune systems that this is really a harmless food?Should all foals be inoculated with midge proteins at an early age?

We don’t yet know, it has taken almost ten years to get this far but we are making progress.In research, there are always new surprises that await, but one thing is certain in science- it always take longer that you think. Yet if everyone works together, one day we willindeed be able to “do something about sweet itch” and develop an effective cure.

Dr Doug Wilson
Dr Doug Wilson is a Lecturer in Virology at the University of Bristol School of VeterinarySciences. His main area of research which has been funded by a research grant from the horsetrust is the immunology of horses with a special interest in the immunopathology of InsectBite Hypersensitivity (Sweet Itch).

The Horse Trust funded Dr Wilson's recent research into sweet itch. The project, whichfinished last year, succeeded in isolating the various proteins in midge saliva that couldcause sweet itch (see "What is it in midge saliva that the horse’s immune system reacts to?"on page 3 for more information).

"I would like to thank The Horse Trust for funding my recent research into sweet itch. Theirfunding enabled me to carry out vital research to better understand the causes of sweet itchand has moved us one step closer to finding a cure for this unpleasant condition," said DrDoug Wilson.

For more information, contact Dr Doug Wilson on Doug.Wilson@bristol.ac.uk

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Comments  

 

0 #3 Kihee 2011-09-01 12:02

I cured my pony of qld itch when he developed it after the incredible rains we had last summer which bought out millions of midges. It takes dedication, science and a few dollars.

First was a wash with permoxin, then a spray with tropical strength aeroguard. A thin cool rug with hood and ear socks. A watered down line of listerine down the rug from between the ears to the dock.

Any sore spots were treated with outback vet and there were quite a few of them.

He hasn't had it since and it was a definite case of qld itch with the telltale raised lumps of the allergy.

I included sulphur and garlic in his feeds until the cooler weather set i.

 

 

0 #2 mona1 2010-04-08 23:16

Great article with in depth detail, yet kept simple for normal people to understand. Maybe,, could it be bred out of horses, if some seem to be more prone to it?? I have inherited a horse that suffered terribly at previous owner's home. We live in a cold climate, the Tablelands, so don't get many nasty little bugs. Horse has recovered beautifully.

 

 

0 #1 keldapark 2010-04-08 21:27

I don't own a horse with 'itch' however, I am very interested in a 'cure'. This article is brilliantly written and extremely informative. Hopefully a cure can be found soon for this uncomfortable problem.

 

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